Higher is better safety. You also might consider two heaters for redundancy (2 rated at 20 gallons or higher would be good).Sweet I’ll check it out , now my next question haha I’m not good with buying the right equipment should I get one rated for 20 gallons or one higher up
I recently had my heater go out on my FOWLR. Since I only have one heater, the temperature dropped to room temperature (70 F) before I noticed the issue and corrected it. Thankfully, the only coral is a toadstool leather and the fish and coral survived the temperature drop.
Redundancy is a good safe-guard.